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Theory & Practiceof

Electrical Insulating Materials

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Electrical Insulating Materials

A material in which a voltage applied between two points on or within the material produces a small and sometimes negligible current. ASTM

A substance in which electrical conductivity is very small (approaching zero) and provides electrical isolation.

A material designed to support a conductor, while not conducting electricity itself.

A material that resists the flow of electric charge.

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Electrical insulating materials are characterized by

High dielectric strength

Low dielectric constant

Low dielectric loss (tan delta)

High volume and surface resistivity, insulation resistance

High tracking, corona & arc resistance

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Dielectric Strength Dielectric strength is defined as the voltage gradient at

which dielectric material fails. It is expressed in volts per unit of thickness.

As a test value it is influenced by many factors.

Electrode geometry. Humidity. Pressure. Temperature. Electrode material. Surrounding medium. Voltage wave form. Voids, contaminants and imperfections. Duration and magnitude of the applied voltage. Thickness of specimen. Time

Influencing Factors

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Dielectric Strength:Quantum mechanics view

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Dielectric Strength Measured as per IEC 60243-1 or ASTM D149. SS electrode is preferred. Electric field intensity at the surface of conductor is

inversely proportional to the radius of curvature. Hence stress is high near the sharp edges. To avoid stress non uniformity, electrode edges should be rounded off.

Voltage source should be free from harmonics transients and surges.

Rapid rise, step by step rise and proof voltage test.

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Dielectric Strength

Units (kv/mm) Air 3 Cellulose Paper 40 Ceramic Paper 10 Epoxy Resin 80 Mica 40 Nomex Paper 28 Polyimide 200 Porcelain 5 – 25 Silicone Resin 100

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Dielectric Constant

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Dielectric Constant The ratio of capacitance of the dielectric to that of free space for the same

electrode configuration, is denoted by dielectric constant of the material.

K = Cd / C V

where K is dielectric constant of an dielectric, Cd is capacitance of dielectric and C V is capacitance of vacuum.

It is also relative ability to store electrostatic energy.

Also known as relative permittivity.

Ratio between permittivity of medium to free space. F = k Q1 X Q 2 / R 2 k = 1/ 4 π ε K = ε / ε0 relative permittivity where F = electrostatic force, Q = charge, R = distance between the charge and ε =

permittivity of dielectric

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Dielectric Constant Polar materials show high dielectric constant.

Capacitor should be made of high dielectric constant material to enable it to store more energy.

While insulating materials for motors & transformer should have low dielectric constant.

High dielectric constant materials dissipate more energy hence lead to higher losses / temp.

Dissipation of energy value is frequency dependent.

Applicable test IEC 60250/ASTM D150.

If two dielectrics are combined in series .Voltage gradient within the dielectric will be inversely proportional to their dielectric constant.

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Dielectric Constant

Dielectric constant of few materials at 50 hz.

Vacuum = 1 Polyethylene = 1.5 Mica = 3 Epoxy Resin = 4 Polyamide imide = 4.3 Fiber glass = 5 Water = 80 Materials having dielectric constant above 10 are not

insulating materials.

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Volume , Surface Resistivity &

Insulation Resistance

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Volume , Surface Resistivity & Insulation Resistance

The total current passing through the insulation may be considered as consisting of two components of current.

One which flows through the volume of insulation that is Iv and other passing over the surface of insulation that is Is.

In practice it is difficult to isolate above 2 components in process of measurement, Hence an over all parameter IR is used to denote the quality of insulation.

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Components of resistance Capacitive current Conductance current Leakage current Absorption current/ Polarization current

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Resistance measurements are affected by

Magnitude of applied voltage. Time of holding applied voltage. Presence of residual charges. Humidity Temperature Impurities dust, grease on surface

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Resistance Measurement

Test method : IEC 60093 / ASTM 257 Measurement is mostly done at 500 dc volts Screening of positive electrode for avoiding

unwanted stray currents. Vol Resistance of insulating materials used for

rotating m/c and transformer has values generally above 10 12 Ohm.cm.

IR value is not directly related to dielectric strength.

Measurement of IR for rotating m/c is done as per IEEE 43-2000 & IEEE 95-2002

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Safety Precaution

Testing is done at 500 volts or multiple of it .Hence some safety measures are must

Windings should be discharged completely before taking measurements.

Charging to discharging time ratio of 1:4 is recommended by IEEE 43.

IEEE 43 also recommends line to neutral return voltage should be less than 20 to begin the testing.

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Guide line for voltage to be applied during IR test on rotating machines Winding rated

voltage. < 1000 1000 – 2500 2501 – 5000 5001 – 12000 > 12000

IR test volt dc

500 500 – 1000 1000 – 2500 2500 – 5000 5000 –10000

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Polarization Index

PI is ratio between IR at 10 minute & 1 minute.

• Low PI ratio (nearly 1) indicates that conduction & leakage currents are much larger than absorption current, which can lead to tracking. Above 2 is safe PI value.

• Less than 1 - Dangerous • 1 – 1.5 - Poor • 1.5 – 2.0 - Questionable • 2 – 3.0 - Fair • 3-4 - Good • Above 4 - Excellent If IR value of rotating machine is more than 5000 mohms. PI

value may or may not be an indication of insulation condition. Then instead of PI DA should be calculated

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Recommended minimum IR for motors

Equipment Min IR aft 1 min at 40 C in mega ohm

Random wound winding manufactured before 1974

R 1 min = kv+1

Random wound winding manufactured aft 1974

R 1 min = 5

Form wound windings and DC armature

R1 min = 100

As per IEEE 43

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Dielectric loss

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Dielectric loss Also known as Tan delta , Dissipation factor or Loss

tangent. Is measure of energy absorbed by dielectrics. 0ccurs within polar materials. Is caused by oscillation/ rotation of dipole. It is a frequency dependent phenomena. When tested at HV it reveals extent of PD activity. With ageing of machine dielectric losses increase. It is temp dependent. Moisture absorption increases polarization hence loss.

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Dielectric loss

IC= capacitive current leading by 90 0

IR resistive current in phase with V

v

I

IIC

IR

δ

ø

Tan δ =Dissipation FactorSin ø = Power Factor

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Dielectric Loss

Dissipation factor = tan δ Power factor = cos δ For small δ, tan δ = cos δ tan δ in range of .001 to .005 is most common. Measurement by IEC 60250 / ASTM D150 Measured on balanced bridge type instrument. Power source should be free from harmonics and any

surges. Test is performed mostly on coils or m/c. Tan delta test is used for condition monitoring of motors.

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Tan delta measurement

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Arcing, Corona Arcing, Corona & &

Partial dischargePartial discharge

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Arcing, Corona & partial discharge

Electrical discharge that partially bridges the insulation between conductors.

These discharges produce heat light & sound. Pressure, temperature, distance of separation and applied

voltage govern these discharges.

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Partial Discharge

A discharge that occurs within the solid insulation without completely bridging the gap between electrodes.

• It is a localized break down. • Occurs in sub microscopic to moderate (fraction of mm)

voids inside dielectric . • Due to lower permittivity of gases electric field within void

is much higher than rest of dielectric. Dielectric strength of gases is poorer than solid dielectric. Occurs when voltage gradient is more than 100 v/mil.• Test method IEC 60270/ IEEE 1434

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Corona

Local breakdown of air at adjacent to conductor is known as corona.

Caused by ionization of air leading to electron avalanche.

Photons are emitted giving rise to glow. Glow discharge I < 1 amp Arc breakdown I > 1 amp

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Arc Resistance

It is ability of insulating material to sustain intermittent to continuous arcing.

12.5 kv is applied between two electrodes at distance of 6.2 mm appox. Failure can be gauged by conducting fine path, tracking or complete carbonization of surface a few specimen burst in to flame.

Extremely high temp up to 12000 k may develop Carbon from carbon brush can lead to arcing in

dc machine.

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Tracking

Formation of permanent conducting path across the surface of an insulation due to presence of contaminants, and in most cases the conduction results from degradation of the insulation itself. It is therefore necessary for organic insulation to be present if tracking is to occur.

1. The presence of a conducting film across the surface of the insulation, usually moisture coupled with dust & salinity

2. A mechanism whereby the leakage current through the conducting film is interrupted with the production of sparks.

3. Degradation of the insulation must be caused by the sparks.

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Tracking

The property of a solid insulating material against tracking is represented by a numerical value known as the Comparative Tracking Index (CTI). This is obtained by using a standard test method. Higher value of CTI Indicates better performing material against tracking.

Test method : IEC 60112

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Degradation of insulation

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Degradation of insulation

Degradation is caused by following stresses.

T hermal E lectrical A mbient / Environmental M echanical

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Thermal Degradation Most recognised cause of gradual degradation and ultimate

failure. Generally insulating materials are poor conductor of heat

which aggravates the problem further.

Losses in conductor : I2 R, eddy current & stray load losses. Core: hysteresis, eddy current losses. Windage Loss. Dielectric heating due to dipole movement & pd activity. • Insulation defects.

What causes normal temperature rise

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What causes abnormal temperature rise in motors

Overload operation. High harmonic /transient currents. Ripples in dc motor input supply. Frequent startup: Large Inrush current. Loose bars in slot which reduce heat conduction. Reprocessed copper may cause localized heating.• Poor brazing may lead to high resistance at joint. • Poor design, Blocked ventilation hole. • Foreign matters deposited on windings.• Winding faults.

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Thermal Classification as per IEC 60085

Old Class New Class Limiting Temp.

Y - 90 0C

A 105 105 0C

E 120 120 0C

B 130 130 0C

F 155 155 0C

H 180 180 0C

N 200 200 0C

R 220 220 0C

S 240 240 0C

C Over 240 Over 240 0C

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Thermal Classification Ability to withstand rated temp for 20000 hrs.,at specific

temp.

Measurement done as per IEC 60216.

End point criteria as per IEC 60216.2 could be: • 50% loss in dielectric strength. • 50% loss in tensile strength. • 5 -10 % loss in mass. • 50% loss in elongation. • 50% loss in flexural strength. • One material can have different temp Index for

mechanical & electrical stresses.

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Thermal degradation quantified

Arrhenius law L = A e B/T

L = Life in Hrs A,B = Constant T = Temperature in kelvin Valid only for 1st order reaction. Valid only at relatively high temp. Thumb rule: life will decrease by 50 % for 10 0 C rise in temperature.

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Thermal Endurance Plot

Temp in 0C

Log

of

T

ime

200 240 260 280

•

•

•

T

100

1000

10000

100000

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Temp index of some of the electrical insulating materials.

Kapton hn • 240 Nomex 410 • 220 Ceramic Paper cequin • 220 Bisphenol A epoxy • 155 Cyclo aliphatic epoxy • 180 Tri functional epoxy • 200 – 220 Silicone resin • 180 – 220 Polyester film • 140 Polyamide imide Resin • 200 – 220 Polyesterimide Resin • 180 - 220

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Mechanical Stress It arises due to moving parts in the electrical machines

Centrifugal force: on rotors, non vibrating, tends to distort insulation.

Magnetic forces :Two magnetic fields from top & bottom bar of each slot, these fields interact and cause bars to vibrate. Similar magnetic force is caused in end winding.

Transient, frequent start up switching on of motor • Vibration due to mechanical movements . • Result of fluctuating temperature in winding with changes in

load.

Differential expansion due to differences in thermal coefficient of components giving rise to abrasion.

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Environmental Stress

Humidity, condensed moisture. Dust , carbon, metal particles & oil

contamination. Salinity in coastal area. Ozone, chemical fumes, oxidation. UV/nuclear radiation, freon

refrigerant gases.

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Major insulating materials

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Major Insulating Materials

Mica Paper Ceramic Paper Nomex Paper Glass Fiber Kapton Film Aramide Felt PVC

Epoxy Resin Polyester Resin Silicone Resin Polyamide imide Resin Polyesterimide Resin Polyester Film Polyethylene/XLPE

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Mica Paper Inorganic material. Silicate of potassium magnesium & aluminium. Excellent arc, corona & pd resistance. Very high temp resistance. High thermal conductivity. Pores of mica permit passage for VPI resin to flow through.

DIFFERENT TYPES

OF

MICA PAPER ROLLS

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Mica Paper Making Process Mica is calcined appox at 680 0 C. Quenched with water & wet ground in to

small platelets. Laid on a porous endless belt. Mica platelets bind by Vander wall forces. Moisture is removed on drying drums. Mica is wound on tubes. Water jets can be used for grinding mica.

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Muscovite Mica Paper Most commonly naturally available mica. Silicate of K & Al – H2KAl3(SiO4).

Usable temp limited to 550 0 C. At 600- 650 0 C chemically combined water starts leaving . Shiny silver colour.

Silicate of K, Mg & Al – [HK(Mg,F)3Mg3 Al(SiO4)3.

Usable temp limited to 980 0 C. Shiny brown colour. Preferred for field coil taping.

Phlogopite Mica

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Aramid Mica Paper

3 – 6% aramid fiber is introduced in mica paper.

Aramid Fiber enhances tensile property and improves flexibility.

It is claimed to have better thermal property as a complete system (aramid reduces mica particle separation) .

Short glass fiber is replacing aramid in recent years.

Glass mixed mica has better impregnation ability. Its use has improved dissipation factor.

Thermal property comparable to aramid mixed mica tape.

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Polyimide film (kapton)

Polyimide is synthesized by aromatic dianhydride and aromatic diamine.

One of the highest thermal electrical & mechanical & physical capabilty.

It has V0 flammability(94UL). Kapton can be used to -269C to +400 C

temperature. Good chemical resistance, It does not

dissolve in most of solvents.

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Kapton Grade

HN general purpose grade VN superior dimensional stability FN coated with FEP film AttributesMT:Thermally ConductiveCR:Corona ResistantCB:OpaqueE :Fine circuitryPST :AdhesionXC :Conductive AntistaticBCL-Y :Printable

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Property of Kapton film hnDensity 1.42

Tensile Strength 230 mpa appox at 23 C

Elongation 72 % at 23 C

Temp index 240 for electrical strength

Melting temp OC none

Glass transition temp 360 – 410 0c can be used at 4000c for few hr

Thermal conductivity 0.12 w/m.k

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Property of Kapton film hn

Shrinkage 2.5% 400c 2 hr max

Flammability V-0 (UL94)

Limiting oxygen 37%

Moisture absorption 50% Rh : 1.8% 100% Rh : 2.8%

Electric strength 200 kv per mm

Volume resistivity 1x10 12 ohm.cm at 200 oCmin

Dissipation factor 0.20 appox at 23 C 1K hertz

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Polyimide film uses

Magnet wire insulation. Slot liner. Flexible PCB base. Wire & cable insulation. Transformer & capacitor insulation. Electronics hard drive, cell phone,

printers & camera.

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Impregnating Resin

Impregnating resin is the key component determining electrical thermal & mechanical properties of final insulations.

They are available to meet different requirements at varying price level.

Resins are used for encapsulation, potting, casting and dipping.

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Impregnating Resin Type Bisphenol A epoxy. Multi functional epoxy. Cyclo aliphatic epoxy. Silicone Resin. Polyimide amide. Polyesterimide. Polyester Resin. Polyurethane.

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Impregnating Resin

Selection Criteria1. Size of machine.2. Operating Temperature.3. Manufacturing Process.4. Health Safety & Environmental

Regulation.5. Working environment of motor

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Impregnating Resin Selection Parameters

1. Resin Viscosity2. Storage & Impregnation Temperature3. Pot life4. Sensitivity to moisture, salinity and

deleterious environment.5. Gel time.6. Low shrinkage on curing.7. High thermal Conductivity.8. Low toxicity.9. Acceptable cure cycle.10.Low shrinkage in curing.

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Impregnating Resin Selection Parameters

11. Thermal Class12. Adequate dielectric properties.13. Low vapor pressure14. Good adhesion with parts of motors15. High flash point.16. Satisfactory resistance to thermal shock and

thermal cycling.17. Ability to cure without formation of water or any

volatile product18. Toughness and resiliency to accommodate

differential thermal expansion specially between core and copper

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Aramid Paper

Generally known as Nomex Paper. Chemically it is aromatic polyamide. Aramide is available in meta & para forms. Meta aramid is known as kevlar which

finds use in bulletproof application. Para aramid developed by Dupont is

known as Nomex. Nomex is available in paper , non woven

and pressboard.

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Aramid Paper

Good dielectric properties. Excellent tear and abrasion resistance. It does not shrink embrittle soften or melt if exposed for

short time at 300 C. Usable up to class 220 operation. Resistant to many chemicals. Suitable for cryogenic use. Unaffected by most of the radiation. It is flame resistant (V0 as per UL 94). Does not Produce toxic reaction with humans or animals. It absorbs moisture , but electrical properties are virtually

unaffected. Good varnish and resin absorption.

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Nomex Paper grades

410 Calendered, Strong, tough good Dielectric property.

411 Uncalendered highly saturable.

414 Similar to 410, more flexible & Conformable

418 Contains Mica, suitable for HV.

419 Similar to 418 but saturable

E 56 In between 410 & 411,high yield

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Aramid paper typical property of Nomex 410 5mil

Weight 116 gsm

BDV (rapid rise) 27 KV/mm

Dielectric constant(60 hertz)

2.4

Dissipation factor(60 hertz)

0.006

Breaking Strength 137 N/cm MD66 N/cm XD

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Aramid paper typical property of Nomex 410 5mil

Elongation 15% MD12% XD

Elmendorf tear 3.4 N MD5.2 N XD

Thermal class 220

Thermal conductivity 0.123 W/MK

LOI 31%

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Typical uses of Aramid Paper Nomex

Rotating machine1. Conductor wrap2. Coil wrap3. Slot liner4. Phase Insulation5. End cap & tubing6. Lead insulation7. Coil support

Others1. Dry type

transformer2. Traction

transformer3. Furnace

transformer4. Speaker coils5. PCB shields

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Ceramic Paper

It better known as inorganic paper branded as cequin & tuffquin.

It comprises of mainly fiber glass. Organic content is 10% appx.

It can operate from sub zero conditions to 250 C on long term basis.

Dielectric values are retained even after long term exposure at high operating temperatures.

Thermal conductivity is high hence windings are generally cooler by 10-15 deg.

Moisture absorption is less than 1%.permitting small drying cycle.

Good varnish absorption enhances heat transfer.

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Cequin property

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Cequin Applications

Slot phase & ground insulation. Layer and ground insulation & core

wrap for transformer. Wire & cable wrap. Battery separator. Capacitor & magnet insulation. Switchgear insulation

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Motor testing

IR at low voltage. It indicates degree of contaminants on insulation surface.

Polarization Index. It indicates condition of overall insulation.

Over voltage test. A/C or DC Test reveals insulation weakness.

Surge comparator for inter turn insulation. Slot discharge test for checking adequacy

of ground connection between sufaces of coil and core. Usually for 6.6 KV & more.

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Motor testing

Corona probe test indicates unusual ionisation within insulation.

Rotor winding impedence test for detecting turn to turn fault.

Power factor of insulation from windings to core , it indicates aging deterioration. Generally applicable to 6.6 kv and more.

Partial discharge test. Indicates aging of insulation.

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Motor testing.

Capacitance test can indicate thermal deterioration or saturation of insulation with moisture.

Semiconductive coating contact resistance. This indicates if coils are loose in slots or if coating has deteriorated.

Wedge tap test indicates loose coil. DC conductivity will reveal if copper

strands are broken or cracked. Black out test measures PD by light

emission.

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Losses in Motors

Losses in conductor(I2 R). Core (hysteresis & eddy current )&tooth pulsation losses. Insulation losses dielectric heating due to dipole movement

& pd activity. Windage loss energy lost in moving the air around in the air

gap between rotor & stator. Friction losses. Rotor slip loss in squirrel cage induction ac motor. Frequent startup raise winding temperature. Insulations defects. Generally insulating materials are poor

conductor of heat which aggravates the problem further. Stray load losses. Brush contact loss for DC machines.

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Causes for Traction Motor failure

Vibration : It is sever for axle mounted TM less for fully suspended TM Each time engine passes thru rail gap (joint) It

sends shock wave. Uneven track.

Dust : Leads to tracking / arcing

Frequent load variation: Excessive heating of coils thereby deteriorates insulation.

Humidity: Tracking, Reduction in dielectric strength, High leakage current.

Power supply fault: High voltage transient can cause dielectric failure specially for ac traction.

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Types of Surgesbase 1 = 230 v

Include Ambient : Effect of Moisture Encapsulation Resin Resins in Detail Silicone/Polyester/Epoxy/PAI